Re: Any ideas on this proposition?

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Dr. Duncan Cadd by way of Terry Fritz <twftesla-at-uswest-dot-net>" <dunckx-at-freeuk-dot-com>

Hi John, Richie, Alex, et al!

>An interesting circuit
>for a tube coil is one recommended by DC Cox.  This involves using
>a primary with many turns, and tapping it at some point.  Then
>another wire is connected to the primary at some different tap point
>and the other end of the wire feeds the base of a resonator.  This
>method eliminates the need for electromagnetic coupling, and can
>match a wide range of impedances.  The tap points are moved
>around until the best tap points are found.  I've tried this method,
>and did obtain about 10% longer sparks.  The resonator is
>connected to the primary circuit though, so I guess it may be
>more dangerous.  When the sparks hit a ground, they seemed
>to be sinking more current than normal.
>
>Any comments on such a circuit?
>
>John


I've copies of this circuit in books on spark transmitters.
Effectively a sort of rf auto-transformer.  It obviously worked well
in the days of spark.  No doubt it would work on conventional spark
coils too (but see comment later).

Yes, it does mean that the output is live to the HT, and that's why
the output sparks seem to be sinking more current - the primary HT is
being conducted to ground via those long streamers!  It's the same
problem with the q&d VTTC - the output arc is live to the power
supply.

As to more dangerous, well, it depends on whether you would want to
insert things into the arc (for fun or measurement - a thermocouple
ammeter might momentarily be fun but alas not much measurement if it
acts as a path to short the psu to ground) and what kind of protection
you have for the mains supply.  I imagine an earth leakage circuit
breaker would not be useful under these circumstances.  Whatever
current limiting is taking place in the power supply is probably the
only thing preventing the mains breakers dropping out :-((

This thought alone has been sufficient to dissuade me from trying it
with a conventional spark-driven TC.  With VTTCs you have a better
chance of controlling the arcs so they don't go anywhere near ground.
Whether this would be a problem with ss coils I don't know.  It sounds
like yet another recipe for dead mosfets !

Dunckx